How Foundries Can Reduce Turbulent Flow with Electromagnetic Pumps
Imagine water being poured into a glass quickly and carelessly: as the glass fills, erratic swirls, eddies, and bubbles appear in the liquid.
That chaotic movement – created by inconsistencies in the velocity of the pour – indicates the flow of water into the glass is turbulent.
This so-called “turbulent flow,” isn’t restricted to hastily poured water – it can also be seen in rapidly flowing rivers, on the surface of choppy seas, and in lava erupting from a volcano.
Turbulent flow is also present in most foundries.
When molten metal is poured from a ladle into a mold, it’s difficult to control the velocity of the stream. Tiny fluctuations in the speed of the pour – coupled with the high density of the metal – can create irregularities and areas of low pressure that entrain air into the casting.
Those irregularities may not seem like a major cause for concern, but they are. The small (and, sometimes, not so small) imperfections caused by turbulent flow can have significant downstream impacts, including costly remelting, increased scrap rates, and part failure.
Defects Caused by Turbulent Flow
Regardless of whether you’re casting doorknobs or automotive parts, filling molds with as little turbulence as possible avoids major defects, including:
Air bubbles are commonly formed in the negative pressure pockets created by turbulent flow – and are something of a double whammy.
Some bubbles remain hollow with an outer oxide skin. Within others, a zero-pressure void is created and the bubble collapses, causing cavitation that can erode molds, coatings and create additional casting defects. Both hollow and collapsed bubbles can threaten the structural integrity of castings.
When hollow air bubbles are trapped in a casting, they tend to rise to the highest point they can reach. These bubbles sometimes ascend to and cause irregularities in the surface of the metal. In other cases, the bubbles remained trapped below the surface, creating weak points in the metal and reducing the casting’s strength overall.
Bubbles that collapse and form metal oxides are equally as harmful. When oxide skins form, they wander randomly through castings, following the flow of the pour. These skins reduce the quality of the metal and make castings more susceptible to cracking and failure.
When too much air or gas gets trapped during casting, microscopic pores can form on the metal’s surface or interior. Like bubbles, porosity can weaken the overall strength of the casting.
Porosity is generally not visible to the naked eye but often becomes apparent during secondary processing. When castings are anodized, for example, the acid used to finish the metal gets trapped in the pores and eventually leaks back out, rendering the parts unusable.
When turbulent flow is extreme, castings may end up misshapen or only partially formed.
Molten metal should travel evenly through a mold, filling all contours completely. If the mold includes hollow areas, the metal should flow uniformly around holes to create perfect circles.
When a pour is turbulent, contours may be unevenly filled, and the metal may form a teardrop – and not circular – shape around hollows.
While some irregularities caused by turbulent flow are obvious to the naked eye, they can be harder to identify in other cases. Detecting those defects with go / no-go tests is critically important in liability castings, like car wheels or engine components.
One such test is a “leak test.” During leak tests, a casting is immersed in a vat of water and compressed air is pumped in. If bubbles emerge, it’s an indication that there is porosity in the casting.
X-rays can also be used to detect bubbles and porosity – but are costly to run.
The Cost of Turbulent Flow
While tests to detect turbulent flow can be expensive to conduct, dealing with damaged castings is even more costly.
Some defective parts can be remelted and recast, but the process involves additional time and labor. Furthermore, some metal will be wasted due to melt loss. Every time aluminum, for example, is melted and cast, approximately 2% of the metal’s mass is lost to oxide.
In some cases, remelting turbulent flow-impacted parts is not straightforward. When oxides form, they will not remelt at the typical 1480°F most aluminum furnaces are heated to. Instead, they must be reheated to 3200°F – an impractical (or impossible) temperature for a majority of furnaces.
Because most foundries cannot, or will not, attempt to achieve such high heat, dross can be sent to secondary remelting facilities – but only as an expensive last resort. Cost aside, many aluminum alloys should not be heated to such extreme temperatures: the high heat can rapidly affect the delicate chemistry of mixed metals.
Sometimes, remelting isn’t even an option. Sand foundries, for example, generally do not remelt parts because debris from the molds would end up in remelted and recast parts. In these cases, if turbulent flow results in unusable parts, the foundry’s scrap rate increases without the possibility of reclamation.
How to Reduce Turbulence During Casting
Most foundries – both high and low tolerance – have some systems in place to reduce turbulence during casting. These include dead-end runners, gating and riser systems.
Unfortunately, those safeguards cannot completely eliminate turbulence if molten metal is hand-poured. Even if a pour is perfectly smooth, molten metal can cool up to 100°F in a ladle during the trip from furnace to mold; the variations in temperature cause poor process control, and the drop from the ladle to the mold will create turbulence.
Fortunately, a solution does exist to help foundries achieve non-turbulent flow: electromagnetic (EM) pumps.
EM pumps are fully enclosed systems installed inside of furnaces that are capable of dynamically addressing the velocity of molten metal every 10 milliseconds – something not even the most expert human hands are capable of.
Unlike mechanical pumps, EM pumps use magnetism to move molten metal – not turbulence-creating vanes that move at high velocities.
Implemented and used properly, EM pumps can help reduce turbulence in gravity fill applications and greatly reduce – or even eliminate – turbulent flow in bottom fill applications.
While it is harder to achieve non-turbulence in gravity fill applications, EM pumps do make the flow of metal less turbulent. The pumps also ensure fewer oxides make their way into castings. Instead of dipping a ladle repeatedly into the furnace (and picking up oxides in the process), the pumps pull clean metal from 16 inches below the furnace’s surface.
When EM pumps are used in bottom fill applications, non-turbulent flow can be achieved. If the correct amount of pressure is exerted, metal flows smoothly and evenly from furnace to mold and leaves the final casting free of bubbles, porosity and other defects.
Reduce Turbulence Today
Since its founding in 1981, CMI Novacast has supplied foundries with EM pumps and related systems (including heated launders, preheat ovens and control systems) that alleviate their most common pain points – including turbulence.
Speak with a CMI Novacast factory expert about automating your casting application and how electromagnetic pumps can help you achieve smooth, non-turbulent flow during casting.